Field
The present invention relates to microelectromechanical devices and especially to an improved pressure sensor structure and a pressure sensor according to preambles of the independent claims.
Description of the Related Art
Pressure is a physical quantity that corresponds to the ratio of force acting on a surface to the area of the surface. A device that can be used as a gauge to measure the pressure is a pressure sensor.
Micro-Electro-Mechanical Systems, or MEMS can be defined as miniaturized mechanical and electro-mechanical systems where at least some elements have some sort of mechanical functionality. Since MEMS devices are created with the same tools used to create integrated circuits, micromachines and microelectronic elements can be fabricated on a piece of silicon to enable various types of devices.
FIG. 1 illustrates an exemplary structure of a microelectromechanical device for sensing of pressure. Microelectromechanical pressure sensors may comprise a thin diaphragm 10 that is spanned over a gap 12 that contains volatile material at a reference pressure. The diaphragm deforms due to difference between the reference pressure and an ambient pressure surrounding the sensor. The diaphragm displacement may be translated to an electrical signal with capacitive or piezoresistive sensing.
A MEMS pressure sensor structure is typically formed of patterned layers of materials. A MEMS fabrication process may involve combinations of layer depositions, optical lithography, etches and wafer bonding. FIG. 1 shows a side view and a top view of the exemplary structure of a microelectromechanical pressure sensor. The exemplary pressure sensor is an absolute pressure sensor that comprises a body structure formed by a planar base 11 and a side wall layer 13. The side walls formed by the side wall layer 13 extend away from the planar base 11 to form a hollow, the depth of which corresponds with the thickness of the side wall layer 13. In this particular category of pressure sensor structures, the hollow is hermetically sealed by a diaphragm plate 16 that extends on the side wall layer 13. A part of the diaphragm plate 16 that extends over the circumferential opening of the gap provides a diaphragm 10 whose periphery is defined by the opening. The diaphragm 10 is exposed on one side to the reference pressure of the gap and on the other side to the ambient pressure. This diaphragm 10 thus deforms in response to a pressure difference between the reference pressure and the ambient pressure. The extent of this deformation may be detected, for example, capacitively by arranging electrodes to elements on either sides of the gap and translating with the electrodes the deformation-induced change in the height of the gap into an electric signal.
A disadvantage of the sensor of FIG. 1 is that it provides a parallel capacitance over the side wall 13. The additional parallel capacitance tends to decrease the relative sensitivity, impair the linearity of the 1/C-function and increase the temperature dependence of the sensor. The structure provides also a path for leakage current and stray capacitance over the outer edge of the side wall 13. This outer edge is located on the exterior surface of the sensor and may be affected by external conditions such as mounting and protection materials, humidity and chemical contamination. These effects may bring a variable part to the total capacitance between the diaphragm plate 16 and the planar base 11, and thereby cause an error when the pressure reading of the sensor is determined.